Embodiments of the claimed subject matter relate to apparatuses and methods for improving the signal quality from dry biopotential sensors. Recording biopotential signals such as electrocardiograms (ECG), electroencephalograms (EEGs) or electromyograms (EMGs) conventionally require the use of a conductive “wet” electrode fixed to the body by an adhesive. The “wet” refers to the fact that a fluid—typically a liquid or gel—provides for a secure, low-resistance electrical connection between the body and the recording devices, ensuring a good signal. These conductive wet electrodes also typically require the use of an adhesive to keep the electrode stationary on the body.
However, the use of electrolytic gels and skin adhesives are messy, can be time consuming to apply, and often are prohibitively uncomfortable for long-term monitoring. Issues ranging from skin irritation from the electrode's gel to dry out and fall-off from daily use result in poor patient compliance and data loss.
In response to the limitations with wet adhesive electrodes, dry electrodes that do not require wet or gel media have recently been explored as alternatives. Before continuing, it is useful to clearly define the two types of biopotential electrodes commonly used. In the context of the present inventive subject matter and the field in general, skin-contact biopotential electrodes can be classified into types, consisting of ‘wet’ and ‘dry’. As previously mentioned, wet type electrodes are commonly found in clinical and scientific applications and specifically require the application of a conductive fluid that is exposed onto the surface of the electrode and the subject's skin on use. The fluid may be an integral part of the electrode package (e.g., disposable ECG electrodes) or applied separately (e.g., standard reusable EEG cup electrodes). In all cases, the purpose of the conductive fluid is to lower the contact impedance between electrode and skin and facilitate signal transfer. A variety of fluids are possible ranging from saline (e.g., skin sweat) to specially formulated gels with high conductivity (e.g., EEG scalp cream). Other wet electrodes are not explicitly in liquid form and may utilize hydrogels, which nevertheless comprise largely of water and hydrate the surface of the skin. It is also possible to use simple tap water which itself is conductive due to it's impurities.
Dry electrodes, however, remove the need for the explicit application of a conductive fluid. In its simplest form, a dry electrode is a bare metal plate. The electrode operates immediately on contact with dry skin. It is important to note that dry electrodes still operate with an inherent amount of ‘wetness.’ In the case of the simple metal plate, ambient humidity from the atmosphere combined with moisture on the skin still help facilitate signal transfer. This is especially noticeable as sweat builds up over time, improving signal quality. The presence of ‘moisture’ is evident even in more sophisticated designs. As an example, micro-needle dry electrodes penetrate skin and leverage the water content under the skin for conduction.
The difference, however, between wet and dry electrode lies with the source of the fluid, it's application and its subjective feeling. In contrast to wet electrodes, dry electrodes do not require the specific application of a fluid or the exposure of ‘wet’ media onto skin. The dry electrode operates even if the skin and the surface of the electrode is dry (e.g., no sweat) and can be reused many times since there is no fluid to deplete. Finally the dry electrode simply feels ‘dry’ to the user since it does not introduce any additional moisture not commonly found on a resting subject. As a result, dry electrodes have many advantages in terms of comfort and longevity making them particularly well suited for long term monitoring applications. The main drawback is the apparent decrease in signal quality due to poor signal transfer, which the described embodiments of the inventive subject matter address.
Dry electrodes operate by sensing the same biopotential signals but through much higher electrode impedances, since conductive gel is not present. Compared to standard wet adhesive electrodes, dry electrodes are prone to a variety of signal quality issues including unstable offsets, high drifts, long-settling times and movement artifacts when used in the absence of adhesives. The majority of signal quality issues with dry electrodes arise from the poor quality of the dry skin-metal electrochemical interface. The standard wet Ag/AgCl electrode enables a consistently high signal quality due to the stability of the Ag/AgCl half-cell potential, which is buffered by a wet/gel electrolyte before contacting the skin. As a result, ionic charges in the body are readily converted to electronic signals through the low contact impedance of the wet gel. Dry electrodes, which typically include a bare metal ‘disc’, form an unstable interface with skin on contact due to the absence of an explicit buffering electrolyte. They must rely on ambient moisture and sweat in addition to parasitic capacitances to conduct potentials from the body. The lack of a buffering electrolyte manifests in drift noise, high contact impedances and susceptibility to movement artifacts. As a result, dry electrodes have not been well accepted for medical use due to the inferior signal quality compared to wet electrodes.
Prior art designs have tried to mitigate the signal quality issues by introducing a wetting agent, typically water, as a means to buffer a non-electrolytic interface. Kopecky (U.S. Pat. No. 3,590,810) describes a sensor comprising a cavity for storing electrolytes covered by a PTFE (Teflon™) membrane to seal the solution and present a conformable surface to the skin. However, the membrane required an externally applied wetting process (unless used on an already sweaty subject) in order to facilitate electrical conduction between the body and the electrode. Similarly, Brun del Re (U.S. Pub. No. 2004/0073104) discloses a design in which a superabsorbent polymer (SAP) is secured within a containment layer (preferred cotton fabric). The SAP is presoaked before usage and emanates moisture across the porous containment layer onto the skin to improve the coupling. However, these designs would still be classified as wet electrodes due to the intrinsic need for a fluid application on both the surface of the electrode and the skin. For long-term use, this poses comfort issues due to exposure of wet surfaces to the skin as well as usability issues arising from the need for fluid application (Kopecky) and replenishment (Brun del Re). Embodiments of the claimed subject matter include methods to implement a truly ‘dry’ electrode that can still achieve a similar contact quality as compared to standard wet electrodes without the use of a wetting agent/process or without introducing moisture onto the skin of the user. The embodiments are suitable for high quality, long-term monitoring applications.